Turns out this is a manufacturing defect. Bose will send me a new set (though I’ll be paying shipping one way). Annoying, since I’ll miss ‘em.

If you know someone with a pair, suggest they look for significant cracks.

Update 2/22/07: Bose sent me a new kit, it took about 2-3 weeks for the exchange. I'm glad to have them back, and pleased with Bose service. I wonder if Minnesota weather might have played a role in their demise; I'll avoid freeze-thaw cycles in the future. Since Bose did well with this one, I'm going to look at buying their new earbud headset ...